DESCRIPTION: The aim of this proposal is to determine what events, at the subcellular level, underlie excitation-contraction (E-C) coupling in cardiac muscle cells. Recent experiments have supported a Ca2+- induced Ca2+ release (CICR) mechanism in which "localized" increases of cytosolic Ca2+ concentration ((Ca2+)i), dependent on Ca influx through sarcolemmal (SL) Ca channels (or the Na/Ca exchanger) during an action potential, trigger the release of Ca2+ sequestered in the sarcoplasmic reticulum (SR). Paradoxically, the clearest examples of the CICR mechanism are observed during periods of cellular Ca2+ loading when spontaneous, propagated increases in (Ca2+)i occur without an action potential and, therefore, without "localized" increases in (Ca2+)i due to Ca2+ influx. This dichotomy of E-C coupling behavior in cardiac myocytes clearly points out that many aspects of the CICR mechanism are not understood at this time. In this project, the applicant will test the hypothesis that "localized" increases in (Ca2+)i is dependent on the degree cellular Ca2+ loading and the organization of SL and SR Ca channels involved in E-C coupling. To test this hypothesis, experiments have been designed to address the following specific questions: (1) how does increased cytosolic Ca2+ buffering affect E-C coupling; (2) can SL Ca channel blockers change the relationship between "localized" increases in (Ca2+)i due to Ca2+ influx and SR Ca2+ release; (3) how does cellular Ca2+ loading affect E-C coupling; (4) does E-C coupling behave differently in atrial and ventricular myocytes? To answer these questions, experiments will be performed on single cardiac myocytes voltage clamped with low resistance patch electrodes while (Ca2+)i is simultaneously measured with fluorescent Ca indicators using spectrofluorimetic and confocal microscopy techniques.